Para-xylene is a useful aromatic material, especially for the production of terephthalic acid, a polyester monomer. It is desirable to produce para-xylene in relatively high yields. Manufacture of aromatic hydrocarbons from acyclic alkanes or acyclic alkenes is well known in the art. For example, U.S. Pat. No. 3,202,725 discloses a process for the manufacture of xylenes containing greater than 95% of the commercially desirable para isomer. The process comprises of feeding to a catalytic dehydrogenation zone various hydrocarbon feeds which include isooctane, diisobutylene, and a mixture of isobutane and isobutylene. The dehydrogenation catalyst comprises 15 to 25% chromium oxide (Cr2O3) on an alumina support composed essentially of eta-alumina. The yield of para-xylene per-pass in the aromatization step is low because of the ease with which the trimethylpentenes are cracked to isobutylene under the reaction conditions. A large recycle stream of the isobutylene is required to be sent back to an acid dimerization step to produce additional trimethylpentane.
The primary commercial source of low molecular weight aromatics (i.e., benzene, toluene, and xylenes) is extraction from catalytic reformate, which is produced in petroleum refining for making high-octane gasoline. Reformate may contain from 20 to 30 percent of C6 to C8 aromatics. High purity aromatics can be removed only by selective extraction because of the overlapping boiling points of these aromatics with other hydrocarbons present in the reformate. Such processes are complex and costly. Isolation of para-xylene adds further complexity. Separation of para-xylene from its isomers is usually done in one of two ways. The more recent method is to use an elaborate multi-valve absorption process using molecular sieves. An older method, still used, is multi-stage fractional crystallization at low temperatures to recover a pure para-xylene fraction.
Another major problem with the separation schemes described above is that the para-xylene isomer of the three possible xylenes is only present in about 20% of the equilibrium mixture. Hence, large volumes of undesired materials are passed through either of the above separation processes to obtain the relatively minor amounts of para-xylene present.
There is a need for an efficient process for the production of para-xylene that avoids the current costly and elaborate separation procedures.